The present invention relates to analytic devices and, more particularly to devices for detecting the presence of chemicals in fluids such as air or other gaseous streams. The device has particular application to the identification of unknown components of a fluid sample, such as toxic, hazardous, or other chemicals.
There are many situations where rapid identification of a chemical component of a fluid sample is necessary. For example, in chemical processing it is frequently necessary to monitor a product gas or vapor or effluent from a plant, in medical diagnosis, it is frequently essential to rapidly determine the concentration of gases in specimen samples of blood or exhaled air, in the analytical laboratory, one frequently encounters the need to measure the concentration of one gaseous chemical in the presence of another and this situation is created in the gas chromatographic detector. There are many other instances where it is necessary or desirable to rapidly and selectively determine the concentration of gases.
It is currently possible to analyze substances selectively using expensive and cumbersome analytic equipment. For such purpose it is usually necessary to obtain a sample of the gas and send it to a laboratory for a remote analysis. This is a costly and time consuming process.
Semi-portable versions of more powerful laboratory equipment have been commercially introduced in recent years. But such instruments have certain inherent limitations. Gas chromatographic devices cannot operate in a continuous real-time monitoring mode. Infrared analyzers require a delicate optical system with a rather long absorption path, which contributes to their bulk, weight and unwieldiness. Furthermore, such instruments must usually be operated, and their results interpreted, by well-trained professionals.
Many existing sensors are incapable of detecting chemical components in low concentrations, particularly where the component is substantially non-reactive. In copending application Ser. No. 585,721, filed Mar. 2, 1984, now abandoned, there is disclosed a sensor which catalytically reacts the component of interest to produce a chemically active derivative product which can be readily sensed. But that device, and most semi-portable or field-usable devices are not selective, but are rather specifically designed for detection of a particular chemical component, and are not designed to both detect and identify an unknown component.
Detectors have been developed utilizing an array of electrochemical sensors, each operated in one or more predetermined modes or conditions, the collective responses being analyzed to provide identification of one of a number of gases. Such detectors are disclosed in copending application Ser. No. 585,699, filed Mar. 2, 1984. But such devices are still capable of identifying only a relatively few components, unless a large number of sensors are used, thereby rendering the device more expensive and complicated and less suitable for portable field use.
Virtually all prior detection devices utilize a sensor which produces a steady-state output signal which changes when the chemical/physical environment changes. The design goal of such devices is generally to eliminate sensitivity to all but one environmental parameter or chemical, thus producing a useful monitoring and measurement tool for that parameter or chemical. But the practical achievement of this goal is extremely difficult. For example, pressure transducers may be susceptible to temperature variations and methane sensors tend to respond to most hydrocarbons. Expensive measures must frequently be taken to minimize this cross sensitivity.
Typical chemical sensors are defined as devices that change an output characteristic (e.g., current, voltage, absorbence, resistance, fluorescence, size, etc.) when exposed to the chemical of interest. The change in response is usually examined at equilibrium or steady-state and the magnitude of the response is related to the concentration. Great care is taken to make sure that the device is designed so that the response only occurs when the chemical of interest is present. This steady-state approach does not provide sufficient data to resolve hundreds or perhaps thousands of chemicals that may be present in a sample using a single sensor, instrument, or sensor array.
U.S. Pat. No. 4,399,684 discloses a gas measuring method wherein a metal-oxide gas sensor is sequentially heated and cooled during exposure to a sample gas. The patent discloses that during such thermal cycling a continuous, concentration-dependent, unique signature for different gas concentrations is produced. This signature comprises a ratio of two samples of the sensor output signal taken at predetermined times during the thermal cycle. This signature yields sufficient information to identify the gas concentration by comparison to standard signatures for known concentrations. But because the signatures developed are concentration-dependent, they cannot be used to identify an unknown component of the sample. The patent implies that the method disclosed may be used to identify an unknown gas, but it gives no explanation of how such an identification could be effected.